Does psi affect multiple sprinklers?

All the guides say to measure psi and gpm. But afterwards, they only say the gpm affects multiple sprinklers. Say a sprinkler runs 1gpm at 35psi and you have available supply of 5gpm, then you can run 5 sprinklers.
However, won't the pressure reduce when you add more sprinklers?

Also read something saying there was a maximum safe gph for each type of pipe. Surely the pipe can handle any flow? It's the pressure that would make leaks?

No, they both affect each other. The only time your pressure will be at it's maximum is when no water is flowing. When no water is flowing the pressure will be the same throughout all the pipes. Once water starts flowing the pressure and volume of flow drop along a pipe's length in a traditional irrigation system.

In the guides they only say to measure static pressure at the outlet faucet. Then you calculate how many sprinklers based on flow at the source and flow of each sprinkler. However, if psi goes down, the flow at each sprinkler will be less than planned?

Yes. They do it for simplicity sake. Otherwise you need to "map out" the other emitters on the zone, know the size of piping, how long the piping, what fittings and what angle they are... Then, the pressure loss will vary depending on the flow rate. So, they just very generally figure if you have this much psi and volume at the inlet then you're good.